Superabsorbents are water-soluble, crosslinked polymers capable of imbibing and retaining, under pressure, large amounts of water, aqueous fluids, especially bodily fluids, preferably urine or blood, by swelling and forming hydrogels. Superabsorbents preferably absorb not less than 100 times of their own weight of water. Further details regarding superabsorbents are disclosed in “Modern Superabsorbent Polymer Technology”, F. L. Buchholz, A. T. Graham, Wiley-VCH, 1998. Owing to these characteristic properties, these water-absorbing polymers are mainly incorporated in sanitary articles such as, for example, baby diapers, incontinence products or sanitary napkins.
Superabsorbents currently commercially available comprise essentially crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers wherein the carboxyl groups are partly neutralized with aqueous sodium or potassium hydroxide solution. These are obtainable by monomeric acrylic acid and/or salts thereof being free-radically polymerized in the presence of suitable crosslinking agents. Different methods of polymerization can find utility here, examples being the method of solution polymerization, the method of emulsion polymerization or the method of suspension polymerization. These different methods ultimately yield water-absorbing polymers in particulate form having a particle diameter ranging from 150 to 850 μm, which are then incorporated in sanitary articles.
However, superabsorbents of this type have a tendency to degrade in color after long periods of storage. The tendency of a superabsorbent to undergo a color transition from a clean, crisp, white color to a honey-brown color accelerates as storage time, temperature and humidity increase. In temperate climes, such as the United States of America and Europe, the rate at which a superabsorbent undergoes color degradation is sufficiently slow such that the superabsorbent, or the article containing the superabsorbent, typically is consumed before a color change is observable to the naked eye. However, in tropical and subtropical climates, such as South America and South East Asia, superabsorbent color degradation is sufficiently rapid such that a color change often occurs before the superabsorbent, or article containing the superabsorbent, is consumed.
The color degradation of the superabsorbent is blamed not only on the initiators used in the free-radical polymerization and remaining in the polymer, examples being ascorbic acid and sodium peroxodisulphate, and inhibitors generally included in the acrylic acid used for the purpose of inhibiting a spontaneous polymerization, an example being the monomethyl ether of hydroquinone (HQME), but also on certain added-substance materials admixed to superabsorbents after their production. Color stability has been found in this context to be particularly affected by oxidizing agents such as, for instance, sodium percarbonate which, as taught by WO-A-2009/066255, are admixed in particulate form to superabsorbents in order that the odor-forming properties of the superabsorbents may be ameliorated. When a superabsorbent containing hygiene article absorbs fluids, for example urine, unpleasant odors are generally quick to form due to the components of the urine, especially ammonia and other alkaline nitrogenous compounds. Bacterial growth can also lead to products which cause odor and/or lead to skin irritation. To address this problem, WO-A-2009/066255 proposes that oxidizing agents such as, for example, sodium percarbonate be incorporated in the hygiene article together with bleach activators. Preferably, the oxidizing agents are incorporated in the absorbent core of a hygiene article together with the superabsorbents. However, it has transpired here that the color stability of the superabsorbents is obviously affected by the presence of the oxidizing agents. Improved odor stability thus comes at the cost of reduced color stability.
The problem addressed by the present invention was accordingly that of overcoming the disadvantages apparent from the prior art.
The problem addressed by the present invention was more particularly that of specifying a water-absorbing composition characterized not only by an advantageous odor stability but also by an advantageous color stability.
The problem addressed by the present invention was also that of specifying a process for producing an advantageous water-absorbing composition of this type.